1. Technical Field
The subject matter includes a method and apparatus for modulating the position of targets used by position measuring instruments such as laser trackers, to result in a modulating nest for tracker targets, that when mounted on an object enables accurate measurement of the position and orientation of the object as well as its velocity. This is accomplished without necessarily requiring communication between the improved target and the measurement system, which permits the improved nest to be used with all tracker systems. Additional utility of the improved tracker target arises from its ability to use solid glass cube corner prisms (retroreflectors) to increase the measurable range of orientations of the object while compensating for the lateral position and distance error caused by them.
2. Background Art
Large volume coordinate metrology instruments such as laser trackers are commonly used as portable Coordinate Measuring Machines (CMMs) not only to measure large objects after assembly, but also to ensure the correct interrelationship (position and orientation) of their sub-assemblies or parts during assembly. They are also increasingly being used as external metrology frames to verify the performance of machine tools and robots in-situ. The measuring instrument is independent of common sources of errors inherent to machine tools and robots, such as geometric errors, errors induced by joints, and thermal errors. The external system permits in-situ, independent, inspection, which eliminates scrap and also eliminates a inspection step typically required at the end of the processing step.
Currently, there are many types of instruments available for measuring the position (X-, Y-, and Z-coordinates, 3 DOFs) accurately. These include optical and electromechanical systems such as laser trackers, triangulation based systems such as indoor GPS (iGPS) systems and photogrammetric systems. iGPS systems use the time at which pulses emitted by a set of emitters are received by the target to triangulate the location of the receiver with respect to that of the emitters. Photogrammetric systems use a model for the optics of each of several cameras to relate the 2D images obtained by different cameras into a 3D scene, of points of interest within the scene.
Laser trackers track retro-reflective targets by driving a two-axis servo system that controls the two orientation degrees of freedom of a laser beam to keep the beam centered about the center of the target. Deviations from center are measured using a detector such as a four-quadrant position sensitive detector (PSD). In interferometric mode (IFM) the distance to the target is measured accurately by counting the interference fringes between light reflected by the target and that reflected internally by a reference mirror. There are also several other technologies for absolute distance measurement (ADM). The two orientation degrees of freedom contain very high resolution encoders to measure the beam orientation precisely. The distance and the two orientations measured are used to calculate the X-Y-Z location of the target.
The two main types of retroreflectors are cube corner reflectors (CCRs) and the cat's-eye reflector. The most commonly used retroreflective target is the Spherically Mounted Retro-reflector (SMR), that comprises of a CCR mounted within a sphere such that its corner coincides with the center of the sphere. The advantages of SMRs include the fact that they can be precisely located inside spherical nests thereby enabling the center of the nest to be measured accurately, and that if a SMR is scanned over a surface, the actual surface can be easily found from the measured points by a constant offset distance.
Different types of nests for holding retroreflectors are known, but all of them hold the SMR in position, at fixed location with respect to the object on which the target is mounted.
Position measuring (3 DOF) systems are typically extended to measure more than 3 DOFs (i.e. one or more angular orientations in addition to the three position coordinates) of an object by either simultaneously measuring multiple targets placed at fiducial points (points whose positions are well known with respect to the object) or by measuring a single target that is sequentially moved to multiple fiducial points. Measuring multiple targets is easy for some types of systems, such as iGPS and photogrammetric systems, with maybe a small penalty in data rate, but reduces the data rate substantially for systems such as laser trackers (possible when operated in absolute distance measurement (ADM) mode). Measuring a single target at multiple positions is easier for systems such as laser trackers, where each tracker can track a target at very high rates, but the target has to be manually moved to multiple locations.
The Leica T-MAC and the API SmartTrack utilize alternate complementary metrology solutions to extend the capability of laser trackers in order to be able to measure 6-DOFs. The T-MAC system uses a target comprised of a standard retro-reflective target as well as multiple LEDs fixed at fiducial points on the target. The system augments the measurement of the position of the retro-reflective target made by the tracker with orientation measurements derived from photogrammetric measurement of the position of the LEDs, to measure the position and orientation of an object. The stated accuracy of orientation measurements is not very high. The API SmartTrack system utilizes an “Active Target™” that uses servos to track the orientation of the laser beam with respect to the retro-reflector and the readings of the encoders on the servos provide two orientation degrees of freedom (pitch and yaw about the laser beam) of the target. The third orientation (roll about the laser beam) is determined using the polarization axis of the beam. Again, the stated accuracy of orientation measurement is not high. In addition, the complexity and cost of these systems is very high.
There are other means of tracking the position and orientation of objects (6 DOF), such as inertial trackers, but these are of low accuracy and precision to be useful for industrial metrology applications where the accuracy is often required to be better than 100 μm. Goszyk (WO 99/30502) describe a method for three dimensional object path tracking using a device that measures the position of one reflector point using three sensors. To measure additional degrees of freedom, additional reflector points are used. The subject matter of the present invention addresses a method for determining the position, orientation and velocity of an object, accurately over a large volume, using only one target point whose position is measured over time.
Runge (DE 10 2008 024 395 A1) discloses a system called A-TOM for measuring 6DOF of quasi-stationary objects using a motor-driven device to move the laser tracker retro-reflector target in a circle. The orientation of the circle is used to determine two orientations (pitch and yaw) and the roll of the object is measured using an encoder. Synchronization of the rotation measurement and the laser tracker used for point measurement is required, in order to determine the orientation of the coordinate frame with respect to which the circle is oriented. This synchronization is achieved using trigger signals directly from the encoder to the laser tracker system. It is claimed that the two can be synchronized to within a few microseconds, thereby making the error in roll determination arising from this to be negligible. However, the jitter in encoder signal (which depends on the precision of the encoder) and other software and hardware delays between triggering and the acquisition of the points by the laser tracker (which may be uncontrollable and variable) can cause this registration between signals to be off, resulting in significant error in roll determination. In addition, the above requirement for communication between target device and the laser tracker causes significant practical difficulties. Most importantly, it is shown later in this document that this method will only work for quasi-stationary objects.
Additionally, since the aim is to track and measure the orientation of objects, it is beneficial to do this over as wide a range as possible. Solid glass corner cube reflectors (CCRs) have a much higher acceptance angle (±50°) compared to the +−20 deg for open glass SMRs). Cat's eyes have an acceptance angle of ±60°, but are very expensive. In the interest of cost and robustness, solid glass SMRs would be preferred. However, to use these, there needs to be a method to compensate for the viewing angle. If the orientation of the object were known from measurement, such compensation can be readily accomplished.
It is an objective of this invention to develop a method and apparatus to modulate the position of one target tracked by one position measuring system so as to accurately and dynamically measure the position, orientation and velocity of an object on which the target is mounted, over a large measuring volume. It is a further objective to be able to determine all six degrees of freedom without any need for triggering or other communication between the modulating device and the position measuring device. It is a further objective of this invention that the modulated target be robust and its acceptance angle made as high as possible.